The present invention relates to plasma assisted processes and apparatus, in particular for performing deposition and etching operations on semiconductor substrates. The invention is particularly directed to the performance of such processes in electrostatically shielded radio frequency (ESRF) plasma sources.
An ESRF plasma source is constructed and operated to generate a plasma in a processing, or plasma, chamber which contains an ionizable gas or a mixture of ionizable gases at a desired pressure. The plasma chamber is usually cylindrical and the gas pressure within the plasma chamber is typically of the order of 0.1 to 10 milliTorr (mT).
The ESRF plasma source typically includes, in addition to the plasma chamber, a radio frequency (RF) oscillator-amplifier that operates at a frequency typically in one of the ISM bands (e.g., 13.56 MHz or 27.12 MHz), a tapped helical or solenoidal coil that is driven by the oscillator-amplifier and surrounds the plasma chamber, and a metal electrostatic shield placed between the helical coil and the wall of the plasma chamber. The oscillator-amplifier includes, or is connected to, an impedance matching network and is typically capable of providing RF power well in excess of 1 kW to the helical coil. The RF power is coupled by the coil into the plasma established within the plasma chamber.
A highly simplified diagram of an ESRF plasma source is shown in FIG. 1. The source is constituted essentially by an enclosure 2 within which a low pressure region containing an ionizable gas can be maintained. Enclosure 2 is surrounded by a grounded RF shield 4 made of a conductive material. The upper portion of enclosure 2 is surrounded by a helical or solenoidal coil 6 having one end grounded via shield 4 and its other end open-circuited. An electrostatic shield 9 is located between the helical or solenoidal coil 6 and the wall of enclosure 2. Electrostatic shield 9 acts to reduce to acceptable levels the amount of RF radiation emanating from the source. RF power is delivered by means of an RF input 5 to helical coil 6 via a tap of coil 6 that is positioned along the length of coil 6 to optimize the ability of any impedance matching network to adjust as required to couple the RF power to the plasma effectively for the intended application under both start-up and run conditions. The portion of helical coil 6 between the tap and the ground end thereof is approximately equivalent, at the operating frequency f0, to a quarter wavelength transmission terminated in a short-circuit.
According to conventional practice in the art, electrostatic shield 9 is provided with a number, possibly 15 to 20, of narrow slots (not shown) which extend vertically parallel to the axis of enclosure 2 and are roughly coextensive, in the vertical direction, with the axial length of helical coil 6. To function properly, electrostatic shield 9 must be provided with at least one well-designed ground connection, as shown in FIG. 1. Preferably, a ground connection is provided at each end of shield 9.
The plasma source shown in FIG. 1 is completed by a substrate support, or chuck, 8 which supports a substrate, such as a semiconductor wafer, that is to be subjected to a deposition or etching procedure. It is to be understood that FIG. 1 does not purport to illustrate the details of such a plasma source, which are already known in the art, and is simply intended to provide an understanding of the basic spatial relations among a plasma source, a power coupling coil, and a substrate support.
The load acting on the oscillator-amplifier is composed principally of helical coil 6, electrostatic shield 9 and the plasma, but also includes various other intrinsic components. The impedance of this load is preferably resonant at the operating frequency f0. Due to the high degree of non-linearity of the plasma, frequency components at integral multiples of the fundamental drive frequency exist with very significant amplitudes. The frequency of each harmonic component may be expressed in the form
fn=nf0,xe2x80x83xe2x80x83(1) 
where n is an integer greater than or equal to 1.
In practice, the electromagnetic energy in a typical ESRF plasma source will simultaneously include components at the fundamental frequency and at one or more of the harmonic frequencies given by equation (1). Through slight variations in the position of the RF tap connection of coil 6, or through other circuit modifications, some control of the harmonic amplitudes is possible.
FIGS. 2A and 2B are plots of the measured frequency spectrum for an inductively coupled plasma source operating at f0=13.56 MHz. FIG. 2A shows the relative amplitudes of the fundamental and harmonic frequency peaks above the xe2x88x9220 db line of FIG. 2B, while FIG. 2B shows the overall frequency spectrum. FIGS. 2A and 2B are reproductions of actual spectral analyzer printout. It is believed that the second peak to the right of the 40 MHz indicium in FIG. 2B is a result of a spurious output. It is obvious that significant frequency components are present at the fundamental frequency f0 and at a number of the harmonic frequencies. It has been found that this is true for values of n less than or equal to about nine. A similar result will be obtained at f0=27.12 MHz.
Plasma chemistry is greatly affected by the so-called electron temperature of the electrons in an ESRF plasma source and it is known that electron temperature depends on the RF power absorbed by the plasma. It is also known that the electromagnetic energy coupled into the plasma in an ESRF plasma source is absorbed in a plasma surface layer having a thickness typically of the order of one centimeter for 1012 electron-ion pairs /cm2 and a drive frequency of 13.56 Megahertz This layer thickness is comparable to the skin depth of the RF frequency in the conductivity of the plasma. The absorption of electromagnetic energy in this surface layer is analogous to the well-known xe2x80x9cskin effectxe2x80x9d in metallic conductors. The surface layer thickness is approximately proportional to the inverse of the square root of the fundamental frequency.
More specifically, the electron temperature is in the region of high power density absorbed proportional to the RF power density absorbed by the plasma. That is, the electron temperature is proportional to the RF power density in the surface layer in which the RF power is absorbed.
It is known that electron temperature can be measured with the aid of Langmuir probes immersed in the plasma, by analysis of the optical emissions from the plasma, or by analysis of the microwave emissions from the plasma. Measurement by analysis of microwave emission from the plasma to determine the electron temperature has the advantage of being non-intrusive and of being usable with various reactive gases as desired that may interfere with the quality of the contact between the plasma and the probe.
Another plasma parameter that is important for practical applications is electron, or plasma, density. It is known that the electron density increases almost linearly with the RF power density in the plasma, i.e., the absorbed RF power divided by the total volume of the plasma. In contrast, the electron temperature is proportional to the RF power divided by the volume of the plasma surface layer.
Electron temperature and electron density both influence the results of plasma assisted processes in different ways. For example, the electron density directly affects the concentration of ionic and neutral species available to react at a wafer surface to produce the desired result. In general, a greater electron density produces a greater process throughput due to a greater deposition, etch, or cleaning rate. However, a plasma process may require an electron density less than some process-dependent value, because the excessive generation of energetic species such as energetic ions or ultraviolet photons may cause damage to the wafer or to semiconductor devices already fabricated or being fabricated on the wafer. Typically, therefore, throughput considerations set a lower bound on electron density and the minimum acceptable process yield sets an upper bound. If the plasma density is to be increased with an attendant increase in processing rate then care needs to be taken to provide a similar chemistry of the gas species modified by the plasma on their path to the wafer. Increasing plasma density necessarily increases the modification of species.
A decrease in electron temperature or in the energy of the plasma electrons decreases the plasma effect on the gas species, therefore increasing plasma density and decreasing electron temperature would allow a higher processing rate with the same chemistry. In reality the situation is a good deal more complicated, but independent control of both electron temperature and electron density provides the ability to optimize process rate and chemistry.
The present invention is a method for generating a plasma having a selected electron temperature, in which electromagnetic energy having components at at least two different frequencies is derived and actively controlled. The derived electromagnetic energy is coupled into a region containing a gas to ionize the gas and create a plasma composed of the resulting ions and an almost exactly equal number of electrons; and a power level for the electrical power component at each frequency is selected in order to cause the plasma to have the selected electron temperature.
The present invention is also an apparatus for generating a plasma having a selected electron temperature. The apparatus essentially includes generating means for deriving and actively controlling electromagnetic energy at at least two different frequencies. The derived electromagnetic energy is inductively coupled into a region containing an ionizable gas to ionize the gas and create a plasma composed of the resulting ions and an equal number of electrons. Plasma by definition is a charge neutral entity while ions by themselves are a beam or isolated charge. The apparatus further includes selecting means coupled to coupling means for selecting a power level for the electromagnetic energy at each frequency in order to cause the plasma to have the selected electron temperature.
In preferred embodiments of processes and apparatus according to the invention, the electron density can be controlled by proper selection of the total power delivered to the plasma, while electron temperature is controlled by supplying the RF power at several frequencies and properly adjusting the distribution of delivered power among the several frequencies.